A series of technological challenges must be overcome in order to transition the current energy economy, based on the consumption of nonrenewable petroleum-based energy resources, into one in which humans sustainably produce, store, and consume renewable energy. With respect to automotive transportation, in particular, first and foremost among these challenges is the unmet need for renewable energy storage devices which are suitable replacements for the internal combustion engine. Rechargeable batteries, and lithium (Li) rechargeable batteries in particular, may be suitable substitutes in electric and hybrid-electric vehicles, but the high cost and limited performance of the conventional batteries available today has restricted large-scale market adoption of this technology. A key component of such batteries which limits its performance is the electrolyte.
In a rechargeable Li ion battery, Li+ ions move from a negative electrode to a positive electrode during discharge and in the opposite direction during charge. An electrolyte physically separates and electrically insulates the positive and negative electrodes while also providing a medium for Li+ ions to conduct between the electrodes. The electrolyte ensures that electrons, produced when Li metal oxidizes at the negative electrode during discharge of battery (e.g., LiLi++e−), conduct between the electrodes by way of an external and parallel electrical pathway to the pathway taken by the Li+ ions. If Li+ ions and electrons recombine, as can happen when they share a conduction path, before both conduct separately from the negative to the positive electrode, no useful work is captured and Li dendrites may form and lead to thermal runaway. In some electrochemical devices, electrolytes may be used in combination with, or intimately mixed with, cathode (i.e., positive electrode) active materials to facilitate the conduction of Li+ ions within the cathode region, for example, from the electrolyte-cathode interface and into and/or with the cathode active material.
Conventional rechargeable batteries rely on liquid-based electrolytes which include lithium salts dissolved in organic solvents (e.g., 1M solutions of LiPF6 salts in 1:1 ethylene carbonate:diethylene carbonate solvents). However, these liquid electrolytes suffer from several problems including flammability during thermal runaway and outgassing at high voltages. Solid state ion-conducting ceramics, such as lithium-stuffed garnet oxide materials, have been proposed as next generation electrolyte separators in a variety of electrochemical devices including Li+ ion rechargeable batteries. When compared to liquid-based electrolytes, solid state electrolytes are attractive for safety reasons, such as not being flammable, as well as for economic reasons which include low processing costs. Solid state lithium-stuffed garnet electrolyte membranes and separators, in particular, are well suited for electrochemical devices because of their high Li+ ion conductivity, their electric insulating properties, as well as their chemical compatibility with Li metal anodes (i.e., negative electrodes). Moreover, solid state lithium-stuffed garnet electrolyte membranes can be prepared as thin films, which are thinner and lighter than conventional electrolyte separators. See, for example, US Patent Application Publication No. 2015/0099190, published Apr. 9, 2015 and filed Oct. 7, 2014, entitled GARNET MATERIALS FOR LI SECONDARY BATTERIES AND METHODS OF MAKING AND USING GARNET MATERIALS, the entire contents of which are incorporated by reference in its entirety for all purposes. When these thinner and lighter lithium-stuffed garnet separators are incorporated into electrochemical cells, the resulting electrochemical cells are thought to achieve higher volumetric and gravimetric energy densities because of the volume and weight reduction afforded by the solid state separators.
Certain solid state lithium-stuffed garnet electrolytes are known. See, for example, U.S. Pat. Nos. 8,658,317; 8,092,941; and 7,901,658; also U.S. Patent Application Publication Nos. 2013/0085055; 2011/0281175; 2014/0093785; and 2014/0170504; also Bonderer, et al. “Free-Standing Ultrathin Ceramic Foils,” Journal of the American Ceramic Society, 2010, 93(11):3624-3631; and Murugan, et al., Angew Chem. Int. Ed. 2007, 46, 7778-7781). However, to date and the best of Applicants knowledge, there are no public disclosures of commercially viable thin film solid state lithium-stuffed garnet electrolyte separators or membranes for Li rechargeable batteries which have a sufficiently long cycle life at high current densities and which conduct large amounts of lithium without forming lithium dendrites. There are also, to the best of Applicants knowledge, no public disclosures of commercially viable thin film solid state lithium-stuffed garnet electrolyte separators which have low interfacial and bulk ionic resistance and/or impedance.
Some of the contributors to bulk and interfacial resistance and/or impedance in lithium-stuffed garnet electrolytes are impurities in the lithium-stuffed garnet oxide, which include but are not limited to secondary phases other than a pure lithium-stuffed garnet oxide which can be found at either or all of the electrolyte's bulk, surface and/or interface with other materials. Resistive secondary phases, e.g., Li2CO3 on the surface or interface of a lithium-stuffed garnet solid electrolyte are also a source of high impedance and poor cycling performance in lithium-stuffed garnet electrolytes. Previously, researchers mechanically processed lithium-stuffed garnet electrolytes to remove secondary phases from its surfaces. These techniques included sanding or polishing the electrolyte surfaces to physically remove surface contaminants. However, these mechanical processing techniques are cost-prohibitive for high volume production, tend to be destructive to the material being processed, and tend not to prevent the formation of new surface contaminants or otherwise stabilize the mechanically polished surface.
There is therefore a need for improved thin film solid state electrolytes and, in particular, lithium-stuffed garnet electrolytes, which demonstrate commercially viable cycle life properties at high Li+ current densities. What is needed in the relevant field is, for example, new thin film solid state ion-conducting electrolytes as well as processes for making and using these solid state electrolytes. The instant disclosure sets forth such materials and methods of making and using the same, as well other solutions to other problems in the relevant field.